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This drawing illustrates how astronomers of the third Sloan Digital Sky Survey (SDSS-III) used quasar light to trace the expansion of the universe. The expansion is shown by the circular disks of increasing area from left to right. From the Big Bang, the expansion occurs rapidly, then slows down, then speeds up again as dark energy pushes apart walls and filaments of galaxies at different distances (purple). As light travels to us from very distant quasars (white dots on the left), it passes through the expanding universe, carrying with it the story of its journey through this expanding cosmic web. Astronomers have measured the expansion of the universe by tracing how quasar light has passed through these structures.

Researchers determine the most precise measurement yet of the expanding universe

By Barbara Kennedy

April 7, 2014

Researchers determine the most precise measurement yet of the expanding universe

UNIVERSITY PARK, Pa. — Astronomers at Penn State and other institutions participating in the Sloan Digital Sky Survey have used 140,000 distant quasars to measure the expansion rate of the universe when it was only one-quarter of its present age. This measurement is the best yet of the expansion rate at any epoch in the last 13 billion years during the history of the universe. Measuring the expansion rate of the universe over its entire history is key to determining the nature of the dark energy that is responsible for causing this expansion rate to increase during the most recent six billion years.

"This observation represents an impressive advance in our attempts to determine the expansion history of the universe," said Donald Schneider, Distinguished Professor of Astronomy and Astrophysics at Penn State and co-author of the investigation. "Forty years ago attempts to measure the expansion rate at the present time were plagued with uncertainties of a factor of two. Now, with these new data, we know to high precision the expansion rate over five billion years before the Sun began to shine." Schneider is the survey coordinator of the Sloan Digital Sky Survey.

The technique of measuring the structure of the young universe by using quasars to map the distribution of intergalactic hydrogen gas was pioneered by the largest component of the third Sloan Digital Sky Survey (SDSS-III), the Baryon Oscillation Spectroscopic Survey (BOSS). New BOSS observations of this structure are being presented at the April 2014 meeting of the American Physical Society in Savannah.

These latest results combine two different methods of using quasars and intergalactic gas to measure the rate of expansion of the universe. The first analysis, by Andreu Font-Ribera of Lawrence Berkeley National Laboratory and his collaborators, compares the distribution of quasars to the distribution of hydrogen gas as a way of measuring distances in the universe.

A second analysis team led by Timothee Delubac of the Centre de Saclay, France, focused on the patterns in the hydrogen gas, itself, to measure the distribution of mass in the young universe. Together, the two BOSS analyses establish that, 10.8 billion years ago, the universe was expanding by 1 percent every 44 million years.

"We have measured the expansion rate in the young universe with an unprecedented precision of 2 percent" Delubac said. "By probing the universe when it was only a quarter of its present age, BOSS has placed a key anchor to compare to more recent expansion measurements as dark energy has taken hold."

An artist's conception of how the Baryon Oscillation Spectroscopic Survey (BOSS) -- the largest component of the third Sloan Digital Sky Survey (SDSS-III) -- uses quasars to measure the distant universe. Light from distant quasars is partly absorbed by intervening gas, which is imprinted with a subtle ring-like pattern. Astronomers now have measured the scale of this ring-like pattern with an accuracy of two percent -- a precise measurement of how fast the universe was expanding when it was just 3 billion years old.

"The past two decades have seen a dramatic revision of our view of the universe, as we now realize that not only is the universe expanding but that the expansion is accelerating," Schneider noted. "Measurements such as the BOSS result, which allow us to track the past behavior of the expansion, are crucial in our efforts to understand the nature of the cosmos."

Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, and the U.S. Department of Energy's Office of Science. This research used resources of the National Energy Research Scientific Computing Center (NERSC), which is supported by the Office of Science.